This invention relates generally to a tomographic scanning system, and more particularly to a method and apparatus for filtering projection data of a helical scan.
Helical computed tomography (CT) has become the method of choice for many routine clinical studies, which requires the application of a helical reconstruction algorithm for the reconstruction of a projection image from the projection data. In at least one known helical scanning system using a CT, an x-ray source and a detector array rotate with a gantry within the imaging plane and around the object to be imaged, such as a patient, while the patient is moved through the gantry in a direction perpendicular to the imaging plane, resulting in a constantly changing angle and location at which the x-ray beam intersects the scanned object. The x-ray fan beam passing through the object is attenuated before it impinges upon the array of radiation detectors. In response, the radiation detectors each produce a signal having a magnitude dependent on the intensity of the attenuated beam. The attenuation measurements from all the detectors over the duration of the helical scan are acquired to produce a scan profile, or set of projection data. The set of projection data resulting from the helix mapped out by the fan beam can be analyzed to reconstruct images of the scanned object.
One method of reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. Reconstruction algorithms for helical scanning typically use weighting functions that weight the collected data as a function of the detector angle and projection angle, and where multiple rows of detectors are used, as a function of row position also. Filtering methods to reduce noise and suppress artifacts are applied to the projection data, thereby producing an overall improvement in quality image. However, some filtering methods, such as z-filtering, requires filtering in the projection-angle-direction, which requires an increase in the number of views and an increase in scan time.
In one embodiment, a method for filtering projection data of a helical scan of an object includes acquiring projection data representing a helical scan of an object, generating a weighting function based on the acquired projection data, determining a scaling function for the weighting function and determining a row-filtered weighting function based on the weighting function and scaling function.
In another embodiment, a system for producing a scanned image of an object includes a computer programmed to acquire projection data representative of a helical scan of an object, generate a weighting function based on the acquired projection data, determine a scaling function for the weighting function, and determine a row-filtered weighting function based on the weighting function and scaling function.